The thread ManectricMadness linked pretty much has also the information on which Pokemon are allowed in it.
Pokemon, that have higher than 4.52% usage in the LC ladder (per 1630 ELO stats) are banned from it. The following Pokemon aren't allowed:
LC UU Banlist: Abra, Chinchou, Dewpider, Diglett, Drilbur, Ferroseed, Foongus, Frillish, Grookey, Koffing, Magnemite, Mareanie, Mienfoo, Mudbray, Munchlax, Natu, Onix, Pawniard, Ponyta, Ponyta-Galar, Porygon, Scraggy, Spritzee, Staryu, Timburr, Trapinch, Tyrunt, Vullaby, Vulpix
As well, you can't use LCBL Pokemon which include:
Carvanha, Dewpider, Farfetch'd-Galar, Scraggy, Shellder, Wingull
This thread contains the viability rankings of the tier. If you click on the Pokemon's name, it also links to a Pokepast including a relevant set, set details and team options.
Pancham is one of the best Pokémon in the tier and fits on just about any team. Fast offensive Pokémon like Scorbunny and Stunky appreciate Pancham removing opposing Eviolites and can make up for Pancham's average Speed. Helioptile can switch into Frillish trying to burn Pancham, as it is immune to Scald and Hex and doesn't mind Will-O-Wisp much. Additionally, it can alleviate Pancham's mediocre Speed with Glare. In return, Pancham can break through Munchlax and Lileep, which trouble Helioptile. The two can also form a pivot core. Frillish benefits from Pancham's general bulk and resistance to Knock Off, as well as it's ability to hit Helioptile. Setup sweepers such as Shell Smash Omanyte can come in safely through Pancham's pivoting and take advantage of the lowered attacking stats to set up safely.
In the link given above, you can also find the other top tier mons, and they also have all sets for it.
LCUU does not have a ladder for itself. So to battle in that format, you have to challenge somebody for a battle.
I also recommend joining the LC UU Discord server for more discussion around the tier.
Hope this helps!