hi, I was wondering if I would fit a 51 bike frame I'm 5,8 and I normally ride a 54 size would the gap be too wide? what problems would arise if I used a smaller bike?

Depends on a lot more than just your height. Go to your LBS and take a test ride.

+1. A lot more to consider other than seat tube length when choosing a frame size. Stack / Reach is two of them.

For instance, I am 6'1'':

-Giant's 2019 sizing chart suggests ML for 5'10'' to 6'0'' and L for 6'0'' to 6'2''.
-Giants 2020 sizing chart suggests ML for 5'8'' to 6'2'' and L for 6'0'' to 6'4''.

Both bikes (2019 & 2020) have however identical specs. This is just to show that you should not only rely on a sizing chart. For my part, I opted for the one I felt more comfortable on.

On a separate note, some people do prefer riding one size down to increase performance.