I absolutely recommend the Alnico/Ceramic mix. I have an Orange AD-30R with a pair of Scumnico's and an extension cabinet with some Scumback Ceramic H75's and they mix phenomenally. The alnico's are creamy and rich while the ceramics are hot and ballsy. However, we are talking about a vintage amplifier here, and lots of very important things should be considered before we go monkeying around with the speaker configurations.
The main thing you need to do is make sure that all of your speakers meet or exceed the wattage requirements by the cabinet itself such that your amplifier doesn't overdrive the speakers to oblivion. The way you figure this for speakers wired in series is by using the following formula:
Wattage for a single speaker >= total wattage of the amplifier/number of speakers
So, given that the bassman is a 50 watt amp you are going to want speakers that meet or exceed 50/4 = 12.5 watts per speaker. Usually that would mean you could theoretically run a 15 watt speaker at each position in the 4x10 cabinet--again as long as you wire them in series. However, I would recommend that you at least double that, and go with 30 to 75 watt speakers at each position. They will have a much higher headroom before breaking up and provide more clarity at higher volumes. All class A/B amplifiers tend to draw more than their rated wattage at very high volumes, and your speakers should be able to handle that.
So in general it's totally doable and relatively easy to do what you want to given that you're okay with placing new speakers back into the vintage cabinet. Also, consider soldering your speakers in instead of using those blasted spade connectors. You'll pick up a decent bit of clarity if you do this (lots of people don't believe me until I let them hear my amp). Again, you'll love the Alnico/Ceramic mix. I do :). Or, you could definitely wire up an external cabinet if you wanted to keep the Bassman in it's original albeit sorta useless splendor.
