The higher I set the threshold on my Decimator, the better it gets, but the sustain gets worse. Will a good compression sustainer pedal fix this. If so, what pedal do you recommend?

I think that would work. I think these pedals go hand in hand; I was reading up on Compressors right now and read that they can get quite noisey when used on the gain channel so it makes sense.
ENGL Fireball
Framus 2x12
Ibanez RG1570 - DM Breeds
Epi Les Paul - Warthog Pup
MXR 10 Band EQ
ISP Decimator
Boss DD3
EHX Holy Grail
EHX Small Clone
Dunlop Crybaby

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I would not expect it to work very well. The compressor works by taking the quiet parts of your signal and amplifying them. The decimator does much the same thing but it makes the quiet parts quieter (reducing the hum in the process, but affecting your sustain). So the compressor might amplify some of your sustain enough to get it over the threshold of the decimator, but it would add noise too, so I don't think you would gain much. I would suggest turning the noise gate down a little bit, it will probably be about the same effect.
If you use a noise gate, your sustain will go down. If you use a compressor, your dynamics will go down. Each is an effect, not an always on deal. Turn the gate off when you're not using your noisy pedals, and maybe think about modding them (A TB mod for a wah is cheap).

If you're using the gate to control a high-gain amp, then you've pretty much lost all dynamics through using high-gain, and then putting it through a noise gate closed tightly, you lose some dynamics because you have to play above a certain level. A comp after the gate would give you a bit of sustain, but also gives you some noise, which if you want a dead silent rig, means putting it behind the gate, defeating what you're trying to combat.

I had the same problem, I ended up turning the gain down on my 5150 combo, from about 7-8 to about 3-4, I was amazed by how much my playing opened up, and I didn't need my gate anymore. Now I'm thinking of getting rid of it, and getting a cleaner amp to push the front end more (Fender-ish or Vox-ish), but then again, my tones have changed a bit.