Replace 6 fuses with 2 breakers?
Assuming the wires are all 12 AWG, this meets Code.  

However, current Code requires dedicated circuits to certain rooms and loads, e.g. bathroom receptacles must serve only bathrooms; two kitchen countertop circuits; dedicated washing machine circuit; yadayada. Your setup as it was, with six circuits was Grandfathered, not obliged to be brought to current Code, however, you cannot make things worse.  So for instance if one circuit once served a bathroom receptacle and 2 others, and now serves a bathroom receptacle and 5 others, that qualifies as "worse" and is not allowed.  

You'll be able to charge a cell phone from any outlet.   You'll be able to run a heater from any outlet.  Two heaters.. good luck LOL.  This is the problem with this setup.  Why would the electrician really cheap out on this deal?

I can think of two possibilities. Some people worry about "old" Knob-n-Tube having arcing faults or ground faults.  I think Knob-n-Tube is awesome, but discretion being the better part of fandom, I would still put K&amp;T circuits on an AFCI breaker because that will detect most potential K&amp;T problems (as well as most potential Romex problems), and GFCI also because that rounds up the last of potential problems, and allows 3-prong plugs on ungrounded circuits.  I suspect the electrician did in fact put them on AFCI, and at $50 each, there wasn't budget for 6 of them. (possibly because you set the budget? ;)

Second, speaking of multi-wire branch circuits, back in the K&amp;T day they positively loved multi-wire branch circuits.  That's where 2 hots share a neutral, and this works if the hots are on opposite poles.  I presume the electrician did attempt AFCI and/or GFCI. Those require a 2-pole AFCI/GFCI breaker, and the installer may not have been able to figure out the intended MWBC topology, or there may have been illicit neutral-sharing that would prevent an AFCI/GFCI from working.  The easy fix is to put all hots which share neutrals (properly or not) on the same single breaker. 

Absolutely, if I had done the job, it would have been six circuits, possibly three multi-wire branch circuits and a spare /3 to avoid hitting NEC 310.15b3a limits whilst giving me some expansion room.  I'm not even thinking of MWBC because the Knob-n-Tube might have that; I just like how it lets you get 8 circuits within 310.15b3a limits. 



I have no idea where you got the notion of a 15A breaker.  Breaker size has nothing to do with circuit function. It has to do with


the wires downline of the breaker
not in this case, but in larger circuits, the type of receptacle
not in this case, but in some cases, the instructions provided with the appliance
not in this case, but sometimes, motor or welder spec


Not counting wires built into lamps, switches or other appliances... If your wires downline contain any wires which are #14, then the breaker must be 15A.  If all wires are #12 or larger, 20A is correct.  Any circuit which serves common NEMA 5-15 sockets (you know the one) must be 15A or 20A.