Nice idea, I was thinking about making guardian myself, but this one is hard nut to crack here are reasons why:
First off main skill - Really great idea but, main skills are hardcoded and they differ from other simple skills by extra features, so the closest thing to your main skill would be to edit phasewalk, but still I am not sure it could work. Also you can forget about slow time, the best slow time we created was to slow enemies movement speed and bullet speed wich was game breaking for multiplayer.
Second thing animation - So far I fit all custom models to Roland’s anim set. To make guardian you would need to create whole anim set for every little animation and animations for each weapon, and sadly I am not really skilled with making animations
And third thing, other skills you listed - Most are doable but some are just not possible due to engine and modding limitations. Here is the list of the problematic ones:
"Movement speed (forward/backward): 3% per rank (left/right): 4% per rank" - there is only one speed modifier for all directions, and backwards is predefinied on character to be for example 20% of original speed, so left/right can be removed from description.
" After killing an enemy, you can double jump for a few seconds" - not possible to make double jumps
"Jump height: 10% plus per rank" - I think this one could be possible, not sure though.
"While jumping, you can attack an enemy below you, dealing 50% melee damage per rank and daze them for 2 seconds per rank" - not possible to make
"Kill an enemy, while your main skill is active, you get +0,2 sec. per level bonus" - I am guessing you mean here +0,2 sec to skill duration, but as far as I noticed its feature from Borderlands 2, and it rather does not work in Borderlands 1.
"You kill an enemy, you become invisible for 1 second per rank" - Not sure if I could make character vanish like guardians teleport, but for sure I can make player untargetable by enemies. In my opionion this skill is a bit OP, its like phasewalk each time you kill enemy
"Bullet deflection chance: 3% per rank" - There is no bullet deflection feature in Borderlands 1
"Weapon swap speed: 10% per rank" - I think its possible to edit weapon swap speed but I am not sure about that
Other skills can be made without any problem. But I must warn you that its impossible to change skill icons. For example if I make your guardian out of Lilith he will have all skill icons from Lilith’s set.