Well the Toolkit belongs to another company, so a sale there would make them happy, and hopefully you as well.
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You are correct that if you have created codes of varying lengths, the shorter codes will always be positioned in the finder above those with longer lengths, regardless of the first character being A or z or 1 or whatever.
The way the auto-fill works in Adagio is that you specify the length of your code, and some character that will be used to auto-fill such as a period. Then when using the finder field to locate a code, you only need to type the first letter or two. When you then press the down cursor key or F5 to open the finder, additional (periods) are automatically added to what you typed, effectively positioning the finder grid to the first code that starts with the characters you typed.
Thus you would want to use the Vendor number changer in the Toolkit for Adagio to change the MRGRO code to something that fits your standardized coding, such as MRG001.
After that, you could then use the Vendor Code format setting in the Company Profile to enforce your 3+3 format (ie AAA999) so that in the future, you would get an error if you tried to create a new vendor with a code such as MRGRO.
And specifically for Payables vendors (not Ledger accounts, yet), there are places where you can look up the vendor by the Name rather than the code, such as in batch entry. Using this does not require changing the code structure, but is limited as not every place where vendors can be selected allow a lookup by Name.