White foam, pink, and blue all work well. White is lighter. Pink and blue have a finer texture. You can probably find white at Home Depot in a 2-inch thick sheet and you can cut the small floats for your plane from a single piece. If you come across gray foam, don't use it. The fumes are too toxic.

You can tack the pieces of foam together before hotwiring with a light coating of 3M77 spray adhesive. 3M77 WILL attack foam if you spray it on too heavily. There is a foam-safe 3M product, but the designation escapes me. With 3m77, lightly spray both sides of the joint. Let it air dry. Then align the pieces carefully and when they touch they are there forever. This joint is adequate if you intend to sheet or glass the floats, and you can hotwire right through the joint without distortion.

After hotwiring, You can glue the foam with epoxy or polyurethane (gorilla) glue. Solvent-based glues ( model airplane glue, PFM, contact adhesive, and zillions of others) will dissolve foam. White glues and aliphatic resins are water-soluble which makes them highly suspect for using on floats, also they take forever to dry since the water in them can't evaporate through the foam. Polyester resin normally used in fiberglass will also eat foam. If you want to glass the floats, use laminating epoxy or water-based polyurethane varnish (Completely waterproof after it dries) .

If you use epoxy to glue the foam together, don't use anything faster than 30-minute epoxy and make the pieces fit so you don't have any puddles of epoxy in the joint. Epoxy gives off heat when it cures. Epoxy cures faster when it is warm. Foam is a very effective insulator. It is possible for epoxy trapped between layers of foam to go into thermal runaway where it gets so hot it melts the adjacent foam. That is really high on the list of bad things. So use slow epoxy in a thin layer and you'll be OK.