Well, again, before I can answer those questions, it helps to know what format and metagame you'll be using this team for. That way we can identify the Pokemon you're allowed to use and the top threats you'll need to build around. However, looking at it right now, you've got a serious weakness to Fire-types, so you'll definitely want to look for something that can deal with those. EQ Dragonite might do the trick as your sixth Pokemon.
I would really like to make a team with smogon rules that uses either the Mew or Celebi that I have and then a Keldeo and Landorus-T. I have an idea for the team with the mew and an idea for the team with Celebi, but I struggled a lot and I still feel like both teams have a lot of problems. So, I am going to post both teams and I would appreciate any help I can get. I'd like to keep the base 3 Mew/Celebi, Keldeo, Landorus-T if possible, but I am completely open to change on the other 3.
A Mew with no Special Attack investment is not going to make very good use of Ice Beam. Everything else about the set aside from the Nature (which I would recommend changing to Careful) screams defensive blanket check, so I would give that Mew Knock Off instead of Ice Beam. That will allow it to provide team support and cripple opponents by removing items.
Stone Edge on Keldeo is a bad idea, period. It's not built to use physical attacks. If you're looking for Flying coverage, which is about the only reason I can think of to use that, Hidden Power Electric will do the job better.
A question: do you use Gengar's Taunt or Charizard's Roost very often? I feel like there could be a bit of wasted potential there.
There's no need to run Defog on Kartana when Mew is carrying it. Another STAB move, ie Smart Strike, would probably be more helpful. Also, I think Fightinium Z would be more helpful than Grassium Z. Leaf Blade already benefits from STAB, and a Fighting nuke is helpful to break things like Scizor.
Celebi is inferior to Mew in the current metagame, so I would probably go with your first team. At the very least, Hidden Power Fire is extremely useful on Celebi; otherwise it's completely helpless against Steel-types.
One last thing: I notice that most if not all of your sets are standard, front-page Smogon builds. There's not necessarily anything wrong with that, because the people at Smogon know what they're talking about, but they don't always work for everyone. The best thing is just to test by yourself. Play a lot of battles, and if you notice that something isn't working or you're never using a certain move, don't be afraid to experiment and change it! Experience is the best teacher.
Mega Sceptile in Singles works best in the UnderUsed tier. I think Empoleon would make a good partner, because the two would have wonderful defensive synergy together and Empoleon can be used either offensively or defensively. I would throw a Choice Scarf Infernape on your team along with that, which gives you a pivot, a Speed factor, a physical or mixed attacker, and completes a Fire-Water-Grass core. Hopefully enough to get you started.
Thanks to LightningRod, Mega Sceptile in Doubles screams to be used on a Discharge abuse team. Loads of Pokemon have Electric immunities and/or Abilities that provide them with benefits from being hit by Electric-type attacks. For partners you could look at things like Raichu, Landorus-Therian, Thundurus-Therian, Electivire, and Tapu Koko.
Okay so I agree with Awesome_Typhlosion on the set comments, but I would suggest bigger changes mostly in the way you are building both your teams. You took sets from Smoogon, thats perfectly fine, I do that too. I would say they will mostly work better than anything you come up yourself, beacuse those sets are carefully discussed before being put in the Strategy pokedex by multiple people, mostly meta experts with huge amount of experience. So if it is not an absolute mandatory for the team comp and strategy, dont try to use anything that is not mentioned in the set commentary for the specific set.
So getting to the point, building a team consists from multiple steps which are described in detail on multiple sites over the internet. But that is quite complicated and if you just want to make a team built around a specific mon (normally you build teams around two mons -"core") it is best to choose mons that synergize well with it.
The thing is, I dont see any strategy behind neither of your teams. You have Lando-T as your defensive/lead mon and rest mostly sweepers (Mew is an execption). This is a definition of an Ultra-offense team, but even than you need some synergy between individual mons. Both your teams lose pretty hard to stall after Gengar, Kartana and Crobat are taken out and they lack any wallbreaking potential.
So what I recommend is to start over. Use Mew offensively instead of defensively (Mewnium Z) and build your team around it. Look into "Team options" in strategy dex if you are using that as your primary source. Also, you wont build a good team from mons you listed. First team is OU, but you lack Tapus who are almost mandatory for any OU team nowadays, no matter if it is stall or ultra offense team. Your second team is mixed OU/UU which from my personal experience wont end well. Celebi and Crobat are both OU irrelevant and you also lack hazard removal so Crobat is most likely getting stomped before it does anything useful.
So in summary: rebuild your team around offensive Mew variant, do not use any lower tier mons unless they have some use for your core team strategy are they are mentioned in "OU viability rankings", think of a core team strategy or build a balanced team (walls, lead, sweepers, support) that has lots of coverage and can deal with all kinds of teams
Hope I helped!
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Agreeing with Agneus here. Just throwing pokemon onto a team won't really be a good team, even if all the pokemon on the team are good within the tier or ruleset. A lot of it is about synergy, how well your pokemon complement each other as well, and you need to make sure you have a clear and well defined win condition. For example, this may be a set up sweeper, but if it is, you need to also think about how can you consistently achieve your sweeper setting up safely.
Also like he said, try to make sure all or almost all your pokemon are from that tier. They're tiered that way for a reason, if you're using UU pokemon in OU they're not in OU because they're outclassed and can't keep up.