Not a problem at all... and again.. thinking ahead... If you think you might get a hankering to do a 10 gallon batch or two, then get the 15 gallon kettle so that 10 gallon batches are easy. My advice is just to look at it economically. Get the biggest darned pot you can get (and a burner to bring it to a boil). You'll never regret it...
Heck, even doing 5 gallon batches in a 15 gallon pot is awesome because you never really have to worry about boil overs. And really, it's a one time purchase. Sure, someday you might decide to upgrade to a 15.5 gallon system (to fill an actual keg). Even if you do, that 15 gallon pot will be just fine for heating strike water. And there are pros who use 15 gallon systems when they start... so that one pot should last you your entire lifetime...
A 5 gallon pot? You'll outgrow it when you go AG. A 6 gallon pot will work, but you'll find it to be a little small. Still, a 6 gallon turkey fryer might be a great budget purchase.
A 7.5 gallon pot? Will work for a 5 gallon AG batch, but you'll outgrow it when you want to make a double batch. And at this point you've already outgrown turkey fryers, so you're either looking at either a keggle (7.5 gallon pony keg or else a 15.5 full size keggle converted for brewing) or else restaurant-type stock pots. It's a very convenient size.... but on the other hand, you've now reached the point that unless you've gotten a really good deal, it won't cost much more to go a bit bigger...
A 10 gallon pot? What's the point? It's a kind of a silly size to get... You won't be able to do an All Grain double batch and you'll HATE doing a double batch without extra space for evaporation, let alone boil overs... It's really not much better than the 7.5 gallon size except that you can theoretically do a 10 gallon batch.
A 12.5 gallon pot? Is good enough for a 10 gallon extract batch, but not nearly enough for an All Grain double batch. And if you're brewing for friends at that point, the cost difference between extract and all grain is not insignificant! Blowing a hundred bucks on extract? Forget about it! (IMHO).
A 15 gallon pot? That'll do a 10 gallon batch, even All Grain, with minimal worries about boil overs and it can even heat enough water for a 15 or 20 gallon batch, which makes it useful for
something even if/when your batches take on extreme proportions... and by that time, I guarantee that a 5, 7.5, or even 10 gallon pot will have been sent to the trash heap, or passed along to another brewer... The 15 gallon pot can be used in a limited capacity as you start a company and brew your own beer for a little while and it's also the ideal size for an all grain double batch... So in my mind, the 15 gallon kettle is the smallest size that will
never need to be upgraded by 99.999% of the homebrewers out there. It's the size I would recommend to anyone who is serious about learning to brew and sharing their beer.
A 20 gallon pot? If you're routinely making double batches (10 gallons) of All Grain beer, this is definitely the size to get (if not a little larger). It will have the space to reduce the risk of boil overs while holding 15-17 gallons of boiling wort. Just try that with a 15 gallon kettle!
And yes... you can ALWAYS go bigger...
But with all that said... there's nothing wrong with starting with a 5 gallon pot and a turkey fryer. Maybe you'll be happy to never go all-grain. Maybe you won't ever want or need the extra capacity. But I've laid out the limitations of each size so you can make the right choice for you. I think it's perfectly reasonable to start out with a pot between 6 and 7.5 gallons and it will take you through All Grain brewing 5 gallon batches just fine. Anything beyond that is a luxury. Unfortunately though, by the time you've reached 6 gallons, you're already beyond the size of most turkey fryer kits, so you're then looking at restaurant supply stores (or "illegally made" keggles)... and at that point, the cost of going a little larger... to ten or fifteen gallons... is not so significant.
So my advice? get a 5 gallon turkey fryer if money is tight and you'll make some damn fine beer. If you've got a little flexibility in your bank account, the 7.5 gallon kettle is a LOT nicer for just a little more money... and if you're getting the 7.5 and space is not an issue, then there's a lot to be said for just getting the 15 gallon one to start with because you will never outgrow it.