Hi Kenny,
Assuming you are talking about an SW Stella; I can tell you that it is totally fine to do so, should your budget be limiting you to a single reel. I did it when i first started because I began jigging, rather than popping, and didn't have the money for both initially. The drawback is that due to the slower retrieval rate you will find, especially when using surface stickbaits, that recovering the line after a sweep will require a fair bit more winding. It will certainly be more of a workout! When you can afford it, get the 18K for popping; you will notice a big difference after a day of casting! There is a slight weight difference too which is negligible really and casting off the 20K spool may be very marginally different in casting distance in comparison because it is wider in diameter.
That is my humble opinion if your budget is tight. Coming from my limited experience compared to many other members of this great forum.
Take care.