Can I connect plain ADSL router to ADSL2+ connection (even with lower speed)?
Yes, but it probably won't be able to connect at the same speeds as an ADSL2+ device - although remember how far you are from your exchange, etc (the usual issues) effects your overall performance.
If you're current device is strictly ADSL, it will likely be incapable of exceeding 8Mbps (again, probably less due to distance to exchange, etc).
ADSL, ADSL2 and ADSL2+ are all different variations on the same standards. The only difference is that which each revision, there is more throughput, more stability and higher possible bandwidth.
ADSL2+ modems are backwards compatible with ADSL in that they are capable of operating at both frequencies (1.1 and 2.2 MHz), so there will be no issues with service., ADSL modems will only sync at the lower frequency which can make ADSL2+ connections potentially unstable or unusable, especially dependent on the distance the of cable path between your modem and the DSLAM. You are likely to have more issues with signal to noise ratio (fluctuations in margins), random drops in sync, excessive training starts when resyncing, lower than usual throughput, higher latency, and of course, reduced bandwidth (you'd have a theoretical max speed of 8.0/1.0 Mbps, but in my experience, most ADSL modems sync at a max of 6.0/768, but that requires the cable path be under a mile in length.
The ISP I worked for would never recommend to customers, nor even mention the possibility, to use ADSL modems with a newly upgraded ADSL2+ connection because of all the problems it can cause.