From what I understand you're trying to build a variable name from variables, which was discussed here: Reference variables within variable names? and currently isn't possible.
What you could do instead which might reduce your code a little, is have a different event for each tool that sets up it's variables, something like so
on getParams do
t_idx = frame
setupTool = "setup_tool_{t_idx}"
call setupTool
end
on setup_tool_0 do
t_name = "camera"
t_cost = 1
t_valid = _c_tool_0
end