Johnson and Starley registers and internal Vent covers

[jo]

I have a new Johnson and Starley and I’d like to upgrade the floor registers. The current ones are metal and old and I have seen some new plastic ones with a nickel top. The design underneath is different but I wanted to check if there is any rules as to what can be used register wise. I also need to replace a vent on the wall and am finding it difficult to get the size 10x16 inches and again wanted to ask of anyone knew if this vent could be reduced in size or if this presents an issue.