You'll have all of the clicks either way, but a properly adjusted RD will keep that click from dumping the chain off of the cassette. As for the index spacing, it shouldn't matter if the you're using the top or bottom seven.
I have a shogun team issue road bike which had 7 speed shimano rsx 100 running gear when I bought it. I managed to get a new rsx 100 sti conversion kit off ebay for Aud $80.00, however this was 8 speed.
I went ahead with the conversion which meant removing the downtube levers and replacing them with the supplied cable adjusters. The extra click is there at the high end, due to cable stretch but the derailleur will not let the chain jump off. Top gear is high enough and I have never accidently tried to engage a higher gear.
I had thought that if this became a problem I could always get an 8 speed rear wheel and if I ever see one on ebay with an rsx 100 hub or even just the hub I will probably buy it but so far I haven't nor has this been a problem