Personally I think the first thing to consider is whether you have the space for two cages. Particularly if you go end up going with a male cockatiel, either one of two things is likely to happen - a) they'll hate each other, or b) they'll like each other just a little too much! In either situation, it's best that you ensure your hen has a safe place to escape to should your new male become either too aggressive or too amorous - and in my experience males can be both at the same time! Breeding type hanky panky can still occur outside the cages of course but they can be supervised or be let out separately to minimise it happening. Hormonally driven aggression is something you do need to consider. My previous female cockatiel (Twinkle) would elicit THE most aggressive hormonal response from my usually mild-mannered male (Fang), and he would chase her round and round and round the house quite relentlessly in his attempts to mate with her, and would also attack me quite viciously when I tried to intervene! I would have to cage poor Twinkle for a while until Fang cooled down, and I most certainly would
never have been able to house them in the same cage together full time. They were generally fine interacting with each other (with supervision!) outside the cage, but the moment Twinkle gave Fang any kind of barely imperceptible signal that she might accept his advances, Fang turned into devil bird and got quite out of control and it could have been very dangerous for Twinkle. These days I have two male cockatiels which you'd
think might get on better together, but their personalities are vastly different and again, I couldn't even think about caging them together.
To eliminate the prospect of breeding altogether, you may be better served getting another female if you can, again perhaps an older one so you can be sure of gender as juveniles all look alike. Even then though they may get on very well, or their personalities may clash and it's more advisable to have separate quarters just in case. In the wild if two birds don't get on they can simply fly away and look for a more suitable companion, but in our homes they don't have that luxury, so I generally recommend separate housing where practicable to help ensure everyone's safety