It seems I am stuck at 30 DT..is this max or I can try getting more?

According to the GECK the maximum DT is 85, however the most one can currently get without mods is 46:

Two ranks of Toughness- 6 (2x3)
Dermal Implant- 4
Full set of Remnant PA: 36 (28 suit, 8 helmet)

Med-X gives DR, not DT, but the two can be used together anyway, and the DR would be applied against whatever damage gets past the DT.

If the game is working properly, you should be able to get over 30 DT, you would simply need better armor, or to take one or two ranks in Toughness, or get the appropriate implant, if you have not done these things already. But you did not state your level, whether you have such perks, or the best armor you've so far found. It may be that the reason you haven't been able to go over 30 is simply that you haven't found armor with better defense yet.

In any case, right now of course the highest amount of damage threshold you can have is limited by what you can get with the best armor in the game and all the defense enhancing perks and implants. It waits to be seen whether future DLC will increase this amount or not. Of course, with mods, it could be possible to get over it, but that's another matter, seeing as you'll only be able to do so if you use those mods, and it's not part of the original game design.

Can you stack Whiskey Rose in with that?

Stonewall adds another +5 DT vs. melee attacks and whiskey rose adds another +5 DT when drinking whiskey

So Stonewall would be effective against Deathclaws, Geckos (not Fire), Night Stalkers, and Cazadors?? If so, I think I'm gonna select that Perk

edit : Nevermind - just checked Wiki - you must be using Melee or Unarmed to get benefit

yes, stonewall plus a couple ranks of toughness, sub-dermal implant and whiskey can add up to +20 DT, also stonewall prevents you from being knocked to the ground

Add Whiskey Rose perk for up to 52 DT and another 50DR from both Med-X and Slasher stacked.

So, with the correct perks and drugs, armour is not needed at all, right?

Well...Until you're out of drugs and booze. Then you have a hangover.

Thing with DT and DR is both aren't that important if you don't take much damage. With both ranks of the toughness perk and the implant I can get by just fine with light weight leather armor most of the time. Even at level 30. Trick is not getting shot in the first place and respond with overwhelming firepower (This Machine), along with popping the key chems, when you do.

Actually, when I have everything needed for max DT, it still doesn't exceed 30. I have Toughness x2, the SubDermal Implant, and a perfect set of the Remnant's PA. Freezes on 30.

Maximum DT attainable is 70
DT 36: Remnant Power Armor with Helmet [28+8=36]
DT 06: Toughnessx2
DT 04: Implant
DT 04: Whisky wish Cassidy as a party member
DT 15: Nerd Rage
DT 05: Stonewall when melee or unarmed weapons are equipped.

With survival at 100 whiskey will give 6DT. So, going by the most extreme setup it's 72DT and 50DR.