I think you should keep it all Catholic.
I do two Bible studies-- one is non-demominational [BSF- Bible Study Fellowship, International] and one is Catholic [a Scott Hahn study downloadable from CatholicExchange.com]. While I enjoy the BSF one-- It has great lectures and I really feel I know the Bible so much better that I ever have in my life, I do feel like I have to "hold back" with certain discussions. It is supposed to be non-demominational and none of the members should know who is from which faith, but it is definitely Prostestant in nature, and I believe has a somewhat anti-Catholic bias.
In the Catholic study I do, I feel so much more comfortable and feel like I can express anything I want to about what I feel God's Word is saying to me. I really appreciate the study because it is tied into the Catechism and to the Mass. It really makes the Bible come alive in a much greater context. Also, I am surrounded by people who basically believe the same way I do, so we can grow in faith together, rather that spend our time argueing about basic points.
Either way, it's great that you are doing a bible study!! God's word is still God's word, no matter the setting-- but I think it would be easier and you will get more out of it with fellow Catholics.
Best wishes to you and Denise and the rest of your group!!