If you are planning to keep them in the same cage, I can only suggest a budgie as you may be able to let two different species interact while under supervision, but living together wouldn't be advisable.
Cockatiels are quite easy going and while they can get on, it's not ideal to put the two together in a cage. Budgies are just a bit too energetic to have around a relaxed cockatiel all the time.
I was like you at first and just couldn't work out how to sex a baby budgie. If I were you, I would read about how you sex them and then look through members posts in the new arrivals section where people are asking for their budgies to be sexed.
Using what you have learnt, try to sex the budgies from the pictures. The more you do it, the better you will become.
If you did get a female budgie accidentally bare in mind that there is no guarentee that a male will get along with Bogart any better than two females. Ultimately it depends on the personality of the birds and how you introduce them to one another.
Getting another bird you will have to get another cage for quarantine and you should be prepared to have this second cage be adequate in size in case the birds don't get along.
Bare in mind, all the negative things I just said are worse case scenario and it's very likely the birds will get on. But going into this with as much information as possible is the best way to go, imo.