My concern is that my current tank water heater will let through, lets
just say for argument sake, 10gpm, and by putting an inline in front of
it, the inline will only let through, say, 3gpm, thus my whole hot
water system will be throttled back to 3gpm. I want to know if small
inlines are limited to a certain flow, or if the flow rating is just a
number that guarentees full heating if the flow is not higher.

I don't want to buy a full size inline water heater since i have a new
regular water heater, and I only want to buy a small inline to put in
front of it,to save money on equipment costs, and to boost the hot
water system a little.