Not directly an SSRI, but I am on an SNRI and have had no bad experience with Kratom. I believe MAOI's are the only drugs that would cause some adverse reactions with kratom. Start off with a low dose though.
I use kratom 3-5 times a day depending on how long I work for, as I need it to get through my long 12 hour manual labor days. I'm not sure about cimetidine, but I'm pretty sure Kratom isnt metabolized the same way as traditional opiates from the poppy.
Also, you're not going to get "noddy" on kratom with your kind of tolerance. Kratom doesn't really make you nod in the same way of traditional opiates anyway. When I first used kratom, I had very little tolerance to where 20mg of oxy had me nice all day. 5-7 grams depending on strain would have me "nod" but it was more of just falling asleep and waking up, not actually that "twilight sleep" state that actual opiates are.
You're still going to be very sick. I know a lot of people on this forum advocate kratom, but I can't stand how often it's toted. I had a 800mg per day oxy habit, and someone told me to "just get some kratom". Let me explain it to you this way. With habits like ours, taking kratom for withdrawal would be like quitting 25mg of xanax a day and being told "just drink one beer, it'll help".
If you're taking any more than 80mg of oxy per day or equivalent, you will probably feel very little relief from the kratom. Maeng Da is also a very speedy strain. I never take it when in withdrawal, as it just gets my blood pressure even worse than usual. Usually when I'm withdrawing, I can find some relief with a mix of 7 grams red bali/7 grams red sumatra. These are the only 2 strains I've ever found relief with when dopesick. I do not recommend taking a green vein like Maeng Da if you have severe withdrawal, with which 1 gram of dope per day would give you.
Honestly, I'll tell you this, if you buy some good UEI and take a G of it, and work your way up til you feel good, you will have completely relief. UEI is 90% mild extract, 10% pure alkaloids. If you're take a full G of it, this equates to 100mg of pure mitragynine, and 10mg of pure 7OH. This equates to about 100mg of morphine or 80mg of oxy per gram of UEI. It is very expensive, at 10 bucks per gram, but hey, 10 bucks is way cheaper than 80mg of oxy, right?