crafts - What size range of diamond will fit this ring?

I found an engagement ring that I really, really liked at a jewelry
store. The price was right with the ring, but their diamonds were too
expensive, so I just purchased the ring. Now I'm looking and loose
diamonds, but then I got to thinking that I should find out what size
range I'm safe to work with.

The guy selling me the ring seemed to think that although it had a
fake 1 carat display crystal in it, it could hold a 3/4 carat diamond
nicely. (His overpriced 1 carat diamonds were out of my price range,
so he was just showing me 3/4 carat diamonds)

Talking with a lady who was trying to sell me a diamond, she said that
she didn't know how good of an idea it would be to try "bending the
prongs" on the setting to fit a smaller stone in a ring that was meant
for a full carat.

Both parties had an incentive to tell me what they told me, so I'm a
little unsure what to think. So here's the info on the ring: it's a
flat band platinum solitaire that holds a princess cut diamond, it
does hold a 1 carat-ish diamond. It has a trellis prong setting. The
design is rather simple, aside from the trellis thing with horizontal
cross beams connecting the prongs midway up (if that makes sense).
The ring size is currently 6.5 that will be sized all the way down to
a 4.

So am I safe to purchase a 0.75 or a 0.85 carat (or a 1.05 for that
matter)? Is this well within the realm of typical gem setting, or am
I pushing it? I could always just get a 1 carat and go easier on the
other aspects, but I would prefer a better cut and less color with a
slightly smaller stone for the money.

TIA,
J