There's lots of information out there about swapping nibs in the Rangas. Teri often uses Sheaffer No-nonsense feeds, which are relatively available, excellent performing but a bit tough to get out of the donor section (without ruining the no-nonsense nib). The hand made feed in the Rangas is also quite poor and your ink flow will really show this.
Once you swap out the feed, you can use Sheaffer No-Nonsense nibs (which look a bit too small for the pen) or other #6 nibs. I've used the inexpensive Knox ones from Xfountainpens.com in my Ranga Duofold. Also senior size Sheaffer lifetime nibs can work as well, tough the early flat-top ones were a bit thick for my Ranga. The OS Balance nibs worked better. Teri has also sold them with dip pen nibs, if you want super-flex.
Two things to know about the Rangas: they are all hand made so every one is a bit different. My two Duofolds cannot exchange caps--not even close. So expect some variation in section inner diameter as well. You may well have to do a bit of minor filing with a rat-tail to adjust for your new nib. Be aware hard rubber is deceptively hard and has been known to dull tools.
Second, the quality of much new ebonite, whether Indian or the 'premium' Ranga ebonite, which I believe is made in China, is not what the old premium Waterman stuff was. Though beautiful, it can be porous or have flaws. If you look closely at the non-premium Rangas, you can sometimes see quite a bit of surface pockmarking and pores. And I'm no longer customizing Rangas because my favorite one developed a long cap crack without being dropped: just from being posted. This should not happen to any new pen. But I suspect it was from a preexisting flaw in the ebonite.
So be careful with yours. It's probably not as tough as those Waterman ripples were when new. But they are beautiful, feel great in the hand and are a great value for a hand-made pen.