If this thread was created to find out if any of us knew a calculation which could determine whether you could add your gar to your current ray tank, ensuring that your tank didn't crash in the process, then again I'll say there isn't a calculation. None of us know.
If I was you I'd simply add the fish and get a handle on your parameters before doing so. I wouldn't feed for the first couple of days either. Observe, and test your water daily.
You're looking for a slight rise in ammonia, though if your bio filtration is good, you may not see anything, it could be seamless, fingers crossed.
If things are fine after a couple of days I'd then feed lightly, still keeping tabs on parameters. If you're lucky you may be able to add this new fish and see absolutely no changes at all in parameters.
If you do see ammonia in the first couple of days of adding the fish, then you'll have to wait for the BB colony to increase and take up the slack, maybe doing a water change too.
I'd much rather take this "hands on" practical approach to adding the fish, rather than looking for some kind of none existent complex algorithm to tell me if it's safe or not!
