The Junior is the same winding, on a shorter stack of laminations (1.25" instead of 1.75"), so the impedance ratio is identical. Mike doesn't keep them in stock, but does a run every so often. The BH-5 is the same winding schedule, with different insulating materials. The leakage inductance and capacitance are both a bit smaller, so the high frequency performance is better. The power handling at low frequencies is reduced by the smaller core - not a problem with the 45.
Transformer cores are overloaded by excess voltage at low frequencies, independent of whether the speaker can reproduce those frequencies. Any overload distortion will intermodulate with sounds that are reproduced, you need to limit the low frequency power applied to the transformer to avoid this distortion. In the case of phonograph (and tape, with NAB eq) there is a built-in limitation, effectively a roll off below 50Hz. You can't always count on this with digital recordings, however. Additional protection is provided by the plate choke and parafeed capacitor, usually in the range of 10Hz-20Hz.
Sorry I'm unable to provide useful specific criteria. I am generally happy with a transformer that can provide the amplifier's rated power at 30Hz with 13000 Gauss of flux, using a M6 grain-oriented silicon steel core. The stock Paramour and Paramour II compromised on this, delivering full power down to around 60Hz, yet was satisfactory to a large majority of customers for many years. At the other end, many people prefer amps which can handle full power down to 20Hz or even 15Hz. For 50% nickel cores, you have to asses it at lower flux density, some use 8000 Gauss and others 10000 Gauss - about 40% and 60% as much power, respectively.
I have tried to balance the plate chokes and output transformers in our kits - a small transformer benefits from a small plate choke which gives a higher cutoff frequency, even though the other benefits of parafeed are reduced by a small choke. But swapping chokes is certainly practical as long as you are alert for the possible effects. The optimal parafeed capacitor value is proportional to the choke inductance, though I have adjusted the production value in some cases - most notably the stock Paramour where 1.25uF is optimal but I used 3.3uF - this deviated from optimum to provide a deeper bass cutoff frequency, at the price of potential transformer overload.
I suppose this muddied the waters more than clarifying things - sorry for that.
