Your grafts will grow normally and fruit

How old is the tree? A seed grown grapefruit can take 10 years to bear fruit.

Hello, I received what I believe is a sterile hybrid grapefruit tree from a friend, it was too thorny for them to keep with kids in the house, she got it from a lady whose son started it from a store grapefruit seed when he was a kid. I believe that explains why it has never flowered or fruited yet, a seed from a store grapefruit probably an infertile f1 hybrid?

Your grapefruit from seed is not a hybrid nor is it sterile. Most grapefruits are able to grow true from seed and should be able to produce fruit just like the one from which it came. The main problems associated with growing grapefruit from seed are:

1) Grapefruits have a VERY long juvenile period before they are able to bloom -- anywhere from 10 to 25 years (or more), depending upon growing season length and other cultural factors, and

2) They need to be VERY large and VERY tall before that can happen. If you chop it back so it fits in your home you essentially delay the production of mature growth. In fact, with constant pruning, it may never bloom. I grew a grapefruit x Poncirus hybrid from seed once and it got to about 14 feet high before it was killed by the cold. It still had not bloomed, but was probably getting very close.

So this is one reason why most citrus are grafted (or started from cuttings) from mature, fruit-bearing wood. Citrus varieties vary in the time it takes them to reach bearing age. Oranges and grapefruit are notorious for the longest period. Key limes, lemons, mandarins, kumquats and calamondins often can bloom much sooner.... 4 to 10 years.

I grafted some fertile trovita orange onto it from a tree that fruits well. It's only been a year since then, haven't seen flower yet but maybe this summer.

Good for you! Your Trovita graft should bloom and bear when it's ready, and as long as the Trovita portion doesn't have too much competition from the rest of the tree.

But I am wondering if the rootstock being this most likely permanently infertile grapefruit, if the hormones etc might never be proper to trigger a bloom in the grafts, or would the grafts operate based on their own design regardless of the rootstock as long as they're receiving nutrition and a place to live on the sterile tree?

Scions don't care about the age or variety of the rootstock (as long as they are compatible and 'take'). In fact most citrus are grafted onto seedlings only a couple years old.

If fertile grafts can bloom and fruit on a sterile hybrid tree I'll put some more on, which was the plan. But if the rootstock being sterile is going to inhibit blossom and fruit of the grafts, is there some kind of hormone or chemistry I could treat it with to trick it to trigger the grafts to bloom & fruit? Please CC your responses to marlajns@yahoo.com in case I don't see them here, thanks very much

Your rootstock should not hinder the scion from blooming or producing in any way.