Only minimum scale really matters - past that you can just cluster engines to get the desired acceleration. Minimum scale for any sort of nuclear thermal rocket is below what you'd want for a manned interplanetary mission, and is thus not relevant.

Solar power plus ion engines is in the 1/2000 TWR range as far as I'm aware. That means millimeters per second squared acceleration of your total craft at best, and means you basically don't save any time over a standard minimum-delta-v Hohmann transfer - 0.001 m/s^2 continuous acceleration gets you 2 AU in ~400 days. It could also do 66,717,283km - the Earth-Mars distance at time of writing - in ~189 days. A Hohmann transfer from Earth to Mars is 259 days. And, of course, the above numbers don't take into consideration matching velocities or escaping Earth's gravity well in the first place. [1] does a good job of describing why the power supply is the primary limiting factor here.

Liquid-core NTRs [0] aren't bimodal, and I'm sort of confused what you'd mean by bimodal here in the first place.

0: http://www.projectrho.com/public_html/rocket/enginelist2.php...

1: http://www.projectrho.com/public_html/rocket/enginelist.php#...