Actually loperamide and buprenorphine (suboxone) are perfectly fine to take together. Both super high doses of loperamide (e.g. 150mg) and more normal lower doses (e.g. 2-20mg) will be fine with buprenorhpine. But unless diarrhea is still a problem after taking the suboxone, why would you take loperamide? There isn't much point other than that.
It sounds a lot like you are going into precipitated withdrawal when you start taking Suboxone. Try waiting longer, 12-24hrs (24hrs is always a safe bet for something like heroin), after your last dose of heroin before taking the suboxone.
Keep in mind that, apart from possibly the first day or three off heroin, no one really needs even 8mg of buprenorphine. I am not telling you not to take that much, just to be aware that is really isn't necessary to avoid heroin withdrawal. 2mg-6mg is really enough to deal with the acute heroin withdrawal.
Loperamide is not nearly as effective as suboxone/buprenorphine for treating withdrawal. Loperamide only addresses gastrointestinal side effects of withdrawal (for the most part) are reasonable dosages (e.g. 2-40mg). There is very, very little reason to take both loperamide AND buprenorphine at the same time, and if you do find you're still having bad diahrea or something, just take a small dose of loperamide (2-10mg, starting off at 2mg and working your way up 2mg at a time, in 1hr increments, until you aren't pissing out your ass every ten minutes). Taking loperamide in conjunction with buprenorphine will constipate you more than you probably would like.
The only way to get loperamide to deal with more significant symptoms of heroin withdrawal is to take huge doses (80-160mg). In such dosages enough loperamide is able to cross the BBB to actually get you a little high (or buzzed, although in heroin withdrawal it probably won't feel like it, it'll just make you normal kinda like the suboxone). There are some nasty side effects of the super high dose loperamide, like bad dehydration, which is why I don't generally recommend it when people are in acute withdrawal, especially from a full agonist short acting opioid like heroin. Loperamide doesn't work as well as buprenorphine at any dose to manage withdrawal from heroin, and the two drugs work in very different ways with significantly different effects and side effects. Bupe is much more effective than lope and safer too.
I would stick to the suboxone/buprenorphine. Like CJ said, and this is really important, avoid the symptoms of precipitated withdrawal you're experiencing when switching onto suboxone by waiting 24hrs after your last dose of heroin (or other full agonist opioid). Like I said, don't worry about having to wait after taking loperamide, taking buprenorphine after it will not lead to precipitated withdrawal.
If you have Suboxone strips, try rinsing your mouth out with Listerine (or other ethanol based mouth wash; fluorine based mouthwash won't work), and directly after spitting the mouthwash out slip the Suboxone strip under your tongue. Doing this will make it dissolve like butter on a hot pan - literally instantly. It will also help your system absorb a bit more buprenorphine compared to without using the mouthwash as well as get it into your system a little faster.
It's worth saying again: Remember to wait 24hrs since your last dose of heroin, until you feel all the effects of bad withdrawal, before taking any Suboxone. If you do that, the suboxone will work really well for you. Some people don't need to wait as long, I've only had to wait like six hours in the past with heroin, other only have to wait 12hrs, but then again yet others have to wait 30hrs. 24hrs is a very safe bet, and no one should need to wait more than 48hrs after their last dose of heroin to avoid precipitated withdrawals with buprenorphine.
If you're just trying to kick, don't take the suboxone for more than 7-14 days, and try to take the lowest dose you're comfortable with. That will make the transition off suboxone the easiest possible, although if you only take it for 7-14 days there shouldn't really be any significant withdrawal from the suboxone other than maybe craving an opioid anyways.
Let us know how it goes. Good luck!