Zezito,
Something was bugging me about your questions, and I now know what it is. Have you considered ripple current. If you use the thyristor circuit that you seem to be proposing, you are going to get large, low frequency, ripples that need a lot of filtering to remove.
Before you go further with your design, I suggest that you calculate your ripple content and then discuss it with someone who has excelent knowledge about your process.
Recently I was involved with the installation of a large cathodic protection system, which is little more than a type of electrolysis process. With this device, ripple would destroy the anodes, so a linear system was used.
It also had an adjustment from 0A to 30A.
The setup was a conventinal step down transformer and bridge rectifier. This was then filtered. It was then run through a variable linear voltage controller with a large transistor amplifier and very large heat sinks. Adjustment was via a trim pot with an ammeter to allow the operator to check the current. It was not PLC controlled, although it had an alarm wired back to a PLC.
To answer your questions:
I would avoid doing this task with a simple thyristor rectifier.
You can control the current for different processes by setting up your system to be fully adjustable from 0 to a maximum value, with the maximum value being at least as high as the highest current you need. The adjust the set point to lower values for other processes.
Current can be measured using a hall effect DC current transducer, and from that you need to calculate the current density. You may be able to even out the current density across the electrode by connecting it at two (or more) ends.
Hope this helps,
Doug