I personally would not use a 28 without a dedicated finder except in a pinch, like if I lost it. The 35 mm frame lines in the 7 don't have so much space around them, estimating the coverage will be difficult. What works slightly better IMHO is using the 85 mm (I think) frame lines as a approximation of the thirds in the 28mm frame. So either budget for a 28 mm finder - the Voigtländer plastic ones are not bad and show up a reasonable prices sometimes, also labeled as Ricoh (for the GR) - or use a 35 mm lens. Of course, if you use an external finder anyway, you could also go wider right away. I, for one, like 28 mm though.