Where are your exosomes coming from? There are many recent publications comparing different RNA isolation kits and normalization controls for mRNA and microRNA analysis. Based on my experience i would also recommend U6, miR-221, miR-103, and let-7a, but it depends on your sample. Good luck!
Many publication shown that house keeping genes expression also varies or may not be expressed. So, for normalization you can choose two or three miRNAs that are expressed constantly in all the samples. In many publications you can see this type of normalization. Remember that there is no universal house keeper in vesicles till now.
in addition to U6 and let-7a you may also consider miRNA one, as it has been reported consistently in EVs (vesicles). Exocarta database lists miRNAs present in EVs and you can find let-7 and miRNA 21 most frequently reported. it can be your choice as housekeeping.