Well of course you can, but most people buying infant carriers get sucked into all this you need a base, you need it to be compatible with the pushchair stuff. Myself included - I'm not being snobbish. It can be easy to end up spending loads on the infant carrier and then you only get a year or so out of it.
There's no evidence I'm aware of that a rear facing combination seat is any less safe than an infant carrier. The infant carrier is frequently referred to as the safest seat type only because it's always rear facing. Going into another rear facing seat isn't any less safe, in fact I've seen some claim that the next type of seat is in fact safer as it's more upright and the more upright you can tolerate travelling, the better. Obviously a newborn can't travel completely upright, but a combination with a good slope is fine for them. There are some brands which don't get a good fit of course. I know the original Britax Dualfix was criticised for this.
Unlike some other car seat enthusiasts I can't get too upset about Which? marking car seats down for being hard to install, considering something like 70% of car seats are installed poorly, it seems to me to be a very important consideration for general recommendations. Anybody who is looking for more detailed information and isn't put off by a tricky install can surely look past that to the safety ratings. For the average user though, it is relevant.
Also, I've never seen this not many colours thing even though everyone repeats it as though it is gospel. I do think they are unnecessarily biased against ERF seats sometimes though - for example they don't seem to mark down seats like the Joie Bold for being hard to install, and I don't think that is any simpler than the 25kg ERF seats. ADAC and Which use the same test results, so if you're using Which to look at the test results, then there's no difference at all. In fact, Which gives you much more info about safety testing than ADAC who just give very basic comments/ratings. The difference is just the verdict which of course may be based on opinion. ADAC often rate a seat "Fail" overall because it contains flame retardants in the fabric - even if it is a brilliantly safe seat with excellent results.