you'll have to find out the correct current travelling through the normal bulbs, know the difference between that and the LEDs and then gt the relevant resistor soldered in the circuit somewhere. its possible, but im not an electrician lol.
yeah theyre road legal if all you're doing is exchanging the bulbs - the brake lights are an added safety feature so "more road legal" if you like - ie brighter and come on instantly so more chance of warning to drivers behind you.
i used to have a set of FK tail lights on my S reg Polo and i loved them, the dual foglight was fun for blinding people behind me who were getting too close or whatever cos they had something like 12 LEDs in each side for the fogs, they were honestly like red headlights on the rear they lit up the road behind... lol