I've been using T-Nation's Full Body workout routine... Should I switch routines to go on my cutting phasE? Maybe more isolation excersises? Help me out plz!

no no no. When cutting you should be using 100% compound exercises, and very low reps.
Instead of ripping up muscles to make them grow, you need to use low reps and hight weight to hold on to the muscle.

So i'll use that same routine? The reps arent too high... Highest is 18 reps, but only 2 or 3 sets... Or how should I balance it out? 2x12, 3x8, 3x10? 2x18? I'd still be doing full workout compound excersises, but could someone help me with the reps and sets?

Stick to 8x3, 6x4, 5x5,

yeah, Lewis is right. For cutting you should focus on rep ranges that make you stronger, because if your muscles arent challenged on a "strenght level" the body thinks you dont need them and will get rid of them.

Ok... So low reps, alot of weight.... Am I correct? Example Routine:

Week #1:
Day 1 (8x3)
Dumbell FLat Benchpress
Military Press
Upright Rows
Squats
Hammer Curls
Deltoid Raises

Day #2(6x4)
Incline dumbell benchpress
lower back extensions
Squats
Military Press
Calf Raises
Tricep Extensions

Day #3: 5x5
Decline Dumbell Benchpress
Squats
Military Press
Upright Rows
Leg Curls
Bicep Preacher Curls

Week #2:
Day 1: 8x3
Military Press
Squats
Incline Dumbell Benchpress
Lower Back Extensions
Tricep Extensions
Calf Raises

Day #2: 6x4
Decline Dumbell Benchpress
Upright Rows
Military Press
Squats
Deltoid Raises
Hammer Curls

Day #3: 5x5
Flat Dumbell Benchpress
Military Press
Squats
Lower Back Extensions
Tricep Extensions
Leg Curls

I would switch the routine around a bit, the order in which I do them... Thats just a draft, what do you guys think?

Also, should I add some HIIT? When should I workout my abs too... Everyday? On NON WORKOUT DAYS? on WORKOUT DAYS? Should I switch routines? Or does the draft look good? Thanks in advance

When I was doing full body I did abs after every workout. They're a regular muscle group, no point in working them everyday. Throw HIIT on your non lifting days, if you don't have time, put it after your lifting.

You're missing some things with the routine too, go back and read the choice of exercises because you don't have any deadlifts in your routine at all! You need deadlifts in there. Put in some pullups, chinups, and/or pulldowns and take out the upright rows. Put in some horizontal pulling like pendlay rows, db bent over rows, seated rows, t bar rows...etc.

I could add dumbell bent rows, but not pullups and chinups because I cant do a single rep!!! I removed deadlifts because I was uncertain if my form was correct.... I could add them again, but aside from that... all good? Add some deadlifts, dumbell bent over rows, and thats it?

You can do negative pull-ups or assisted pull-ups as an alternative, you really should be doing one or the other.

How are negative pullups done? Jump to the position then slowly let go? If I do pullups, how many? Same parameters as the rest of my workout?

Yeah, get like a chair or something to boost you up and slowly come down. Find a number right for you so that you can do as many sets as you are doing for the rest of your workout.

Alright thanks guys...

stick to the pull ups. start with many sets of low reps and keep adding as slowly as you need to.

when i started them i could do 6 narrow grip at most and wide pull ups were a distant hope. 6 months on i can do a good 5x5/4x6 wide grip and 3 sets of 12 narrow grip.

it takes some time and patients and you'll feel a bit of an idiot doing single reps but its worth it as there's not may exercises that hit as many muscles like pull ups do.

pullups are harder than chinups. i started with chinups (i could only do 4) because i couldnt do even 1 pullup. i built my chinups so i could do 13 in my first set then i switched to weighted chinups/pullups.