The gaps will get bigger as time goes on. I'd address the problem now...
I build my racks with the support on the bottom instead of the rail type, just for this reason. My tubs actually rub on the screen, I like them that tight.
The other major problem you'll have with the gap is, it gives the rats a way to chew the top of the tub... Then you'll end up replacing chewed tubs all the time.
Even a weaned rat "can" get out of the small gaps. Most won't, but they can... Then, you're looking at chewed water lines and flooded bins...

Just my .02.