well, the template is mostly for Wild pokemon, but you can catch them, and i will probably make a featto turn a pokemon into a Delta pokemon.
This is just a rough idea, i would still need to test it, and it might not even work with Starks rules.
If a pokemon Is in its first stage It gets rid of its Type(s) and chooses a new one (If a pokemon had 2 types it still only gains 1 type) and gains that as their type, They also pick up to three of their attacks that they had STAB's for and change them to their new type.
When a pokemon evolves it retains its type, and if its evolving into the last pokemon in its species evolutionary line it can chose a second type to take also. Since Single stage pokemon are always at the end of their evolutionary line, they gain a second type as soon as they gain this template (so a female combee that evolves can take a second type, but a male one can't since they aren't at their species final stage of evolution)
When a pokemon would learn a new move that is their old type, they can choose to instead take a move of their new type.