Yeah I have heard some people report raised ammonia levels from Osmocote, but I believe those are from using too much or maybe the granules making it's way up above the substrate and probably releasing the ferts much much faster resulting in the elevated ammonia levels.
However there are tons of reports of success. My strong advice would be to just start off using a little (under-dosing even) and then add more based on how your plants are doing (if they look a little deficient, then add some). Better safe than sorry. You can always add them in pretty easily, but not so easy to take out. I use the 00 capsule form of O+ and push them in 1.5" or so into the substrate. Pushing them deeper might be safer if you are worried, but I haven't had any problems, then again I don't use very much of it at the moment.
I would suspect all root tabs would cause this ammonia problems if overdosed, I believe there is just more chance of it happening with O+ since it contains higher levels of/more nutrients vs other root tabs.
Also keep in mind Osmocote release faster or slower depending on the temperature.
60*F lasts 1-3 months before the nutrients are depleted and need replacing
70*F lasts 3-4 months
80*F lasts 2-3 months
90*F lasts 1-2 months
Keep in mind though those are based on dry applications (land gardening), not sure if the rate of release is affected much in aquatic use.
There is this thread on Osmocote and it affecting ammonia levels (short story, I think O+ can elevate ammonia levels if overdosed, so just don't overdose and apply it properly)
Burr also has this interesting thread on if root tabs with water column dosing makes a significant difference vs just water column dosing. Take a look if interested.
But yeah, I say give Osmocote+ a try, it's cheaper than what you are using and better as it does have a lot more nutrients you plants want that you aren't fully getting with the clay, potash and dolomite. Not sure if you still need a GH booster though for your water params.