Although having multiple device connectivity may be a positive thing, I suggest you look at the characteristics of the devices you have in mind, as there are great differences in the practical identification ability of each one.
Check this video out and you can see how some readers really stand out from the average:
The hardware vendors have their own software development kit and may, in occasions, give it out for free. Unfortunately, these are usually underperforming as compared to general expectations.
Of all biometric technologies, for time and attendance the most practical is fingerprint technology, besides, it is the easiest to implement and cost to benefit ratio is highest.
For multiple device support, you should look into a software development kit by a vendor that has this already embedded in their offering. At minimum you will see three or more devices supported and in cases over 10. But be careful, numbers are not everything and local availability and support for that hardware are critical.
id3 Semiconductors has a MINEX certified extraction and matching algorithm and supports some good quality readers on the SDK:
You may also want to consider utilizing an application specially designed for delivering biometric identification to existing applications. There are apps out there designed solely for the purpose of identifying users and turning the records to Time and Attendance applications for further processing. The advantage of this method is that you only have to configure the add-on software and not worry about spending time in biometric development. Also, they usually offer many advantages that make it very convenient, hence getting a faster ROI (without any upfront investment).
Check this one out:
In any case, if you decide to fully integrate from scratch plan for four to six months for full development and more for testing.