The speaker currently in your amp isn't necessarily 8 ohms, you could buy/borrow a multimeter and check it if you really wanted to know. It may well be 4 and just a terrible speaker which is why you're struggling to hear yourself. If it is a 4 ohm speaker and you add an 8 ohm cab, you'll be running at 2.66 ohms and risk damaging your amp (1/8 + 1/4 = .375, 1/.375 = 2.666).
Seems like the easiest solution to me would just be to replace the current speaker with an 8 ohm Celestion or other quality speaker with a high sensitivity if it's more volume you're after. This should help your sound cut through more and you can add the extra 8 ohm cab afterwards if you still need to push more air.
Edit:- You can mix and match speaker wattages, remember that the maximum you can push into them is the wattage of the lowest rated speaker * the number of speakers. My guitar cab runs 2 Celestions , a Hot 100 (100 watts) and a Vintage 30 (60 watts) so it can handle 120 watts total.