It's okay to want to do more than just strength training. The thing to realize, however, is that the initial period where you start lifting heavy can be very productive if you focus on it, and much less productive if you try to chase two rabbits at the same time.
StrongLifts, like Starting Strength and other 3x5 or 5x5 programs, is based on linear progression, which is made possible because of your status as a novice. You add weight every workout. Once it starts getting heavy, it gets very intense, very fast. Adding things to your weekly workout routine--supplementary lifts, running, rowing, long hikes, playing judo--will dramatically speed up the process of stalling out.
Official StrongLifts Advice
Mehdi, author of StrongLifts, addresses the question of extra conditioning in the first "5x5 Report" ebook. It's on page 57 of my copy:
If you somehow need to do conditioning work (I never do it), then do NOT add it on your
off days. HIIT is way too stressful because it requires you to go all out to be effective. If you do it on your off days anyway, you'd never have any recovery and you'd quickly be
stalling all over the place. My recommendation: wait until the first 12 weeks are over before you even think about doing extra conditioning work, so you have some foundation of
cardiovascular fitness first. Then you can add it, but at the end of your StrongLifts 5x5 (yes that's hard but the only way), never more than 2x per week, and spread it out.
Rowing 20 to 40 minutes definitely qualifies as a significant amount of work to put on top of a novice's linear progression. Devoting your body's recovery resources to attributes other than strength (such as significant rowing) means that your strength will hit a wall faster than if you buckled down and just lifted. If that's OK with you, great! It's perfectly fine to work on multiple things at a time.
It would also be a fine approach to put rowing on the back burner. You could put a light 5 minute session before your lifting sessions as a warm-up. You could also dial down the length of the rowing sessions significantly and do a high-intensity 3 to 5 minute row at the end of your lifting sessions, which would minimize its impact on your lifting progress while maintaining good metabolic conditioning effect.