RE: can I mix 5 min with 30 min?
Weeeeell you might, if you were lucky, but I would not do it.
Bit of theory on how epoxy resin works.
You start with a Resin to which you add a catalyst. This starts the reaction which causes the resin to cure [ harden ]. In one case that I investigated the time to the first stage of 'setting' was 16 hours. N.B. at this stage the epoxy is said to be green and is not yet at full strength.
It is possible to speed this up with an accelerator. There are different chemicals which will do this.
It is possible to slow it down with a retarder. Again different chemicals are available.
It is likely that both the 5 min and the 30 min contains accelerator. If it is the SAME accelerator then yes the 'setting time ' should be somewhere between the 5 and 30 mins. But if the accelerators are different then it could all go horribly wrong.
I am mostly Scottish and even I buy 5 min [field repair box], 12 min [general construction], and 30 min. epoxies.
I try to buy from suppliers with a high turnover and discard all epoxy after a year. Epoxy has a shelf life.